The goal is to evaluate the role of serum human thyroglobulin levels (hTg) as predictors of tumor recurrence in thyroid cancer patients. A patient base of 57 subjects has been recruited for this longitudinal study. Each patient has treated thyroid carcinoma. All patients entered into the study have had initial total body iodine scans and serum thyroglobulin levels and have now extended beyond 2.5 years with subsequent serum thyroglobulin levels. Only two drop-outs have been experienced to date. Of the patients entered into the study, 19 have had positive total body iodine scans and 36 have had negative scans. Of the positive scan group, mean serum thyroglobulin is 366 plus or minus 209 ng/ml, whereas in the negative group, the mean thyroglobulin is 7.8 plus or minus 0.8 ng/ml. It was anticipated in the original application that this study would require at least 4 years of longitudinal studies prior to achieving statistical significance because of the slow rate of thyroid cancer recurrence. However, it is clear that in patients already evaluated longitudinally, serum thyroglobulin levels do reflect total body iodine scans. We now have patients who have shown appropriate thyroglobulin level changes consistent with total body iodine scans becoming negative. Of the 10 patients with positive total body iodine scans who have converted to negative scans, the mean serum thyroglobulin change has been from 129 ng/ml (positive scan) to 6.9 (negative scan). These observations are highly significant. We are now anticipating patients who will convert from negative scans to positive scans to determine the effectiveness of the thyroglobulin assay to show a concomitant increase in serum thyroglobulin. Thus, serum thyroglobulin can reflect and predict total body iodine scan data. (2)